The present invention relates to cardiac pacemakers. More particularly, the present invention relates to protecting implanted telemetry or other sensitive circuits within a pacemaker from high level signals, which high level signals could damage or otherwise interfere with the proper operation of the telemetry or other circuits. Even more particularly, the present invention relates to protecting the ECG telemetry circuits of an implanted pacemaker wherein the ECG information to be telemetered is sensed on the same leads used to provide stimulaltion pulses to the heart.
It is known in cardiac pacemaker art to include telemetry circuits within an implanted cardiac pacemaker. Such telemetry circuits allow selected information to be telemetered out of the implanted pacemaker to an external receiver. Early telemetry circuits limited the telemetered information to the status of the pacemaker's circuits, primarily the status of the battery used to power the pacemaker.
In recent years, with the advent of demand-type programmable pacemakers, it has become common to include in the telemetered information from the implanted device the ECG (electrocardiogram) signals sensed by the pacemaker leads that are positioned in one or more chambers of the heart. (It is noted that in a demand-type pacemaker, stimulation pulses are provided to the heart only when the heart does not contract or beat by itself prior to the expiration of a predetermined timed interval. This predetermined timed interval is typically called the escape interval, and it is measured relative to the last or most recent heartbeat or contraction. Where the heart contracts or beats on its own, no stimulation pulses need by generated by the demand pacemaker. This action of not generating stimulation pulses except when needed advantageously conserves power and frees up the pacing lead to function as a sensing lead.)
Further advances in pacemaker art in recent years have significantly reduced the size and weight of implanted pacemakers. Some of this reduction in size has resulted from being able to use smaller batteries. Being able to use smaller batteries, in turn, has been made possible in significant part by the use of highly efficient circuit designs that minimize power consumption.
One circuit configuration that has contributed to efficient circuit design is the use of a positive ground circuit configuration. In a positive ground circuit, all of the circuit potentials are more negative than the reference potential (ground) of the environment in which the circuit is used. In the case of an implanted pacemaker, which typically provides a negative stimulation pulse to the heart, the generation of such a negative stimulation pulse is more efficiently accomplished using a positive ground system. (This efficiency results because most of the circuit components used to generate the negative stimulation pulse remain off--not consuming any power, or very little power--except when the stimulation pulse is generated. Because the stimulation pulse is a narrow low-duty cycle pulse, these components thus remain off most of the time.)
A common element used in a positive ground circuit configuration is a P-channel MOSFET (metal-oxide semiconductor field effect transistor) switch. P-channel MOSFET's, as explained more fully below, remain off unless a negative voltage is applied between the gate and source thereof. (In contrast, an N-channel MOSFET remains off unless a positive voltage is applied between the gate and source.)
It is known in the pacemaker art to use a P-channel MOSFET switch to selectively connect a pacing/sensing lead to an appropriate telemetry circuit. By turning such a switch on during those time periods when the lead is to function as a sensing lead (rather than as a pacing lead), any signals sensed by the lead, such as ECG activity, can be telemetered out of the pacemaker.
When a negative stimulation pulse appears on the pacing/sensing lead, and when all voltages appearing on the pacing/sensing lead are monitored at a buffered monitoring point, a high level positive voltage signal appears immediately subsequent to the negative voltage signal. (This positive-following-negative voltage effect is created in large part by the AC coupling capacitors that must be used in any monitoring circuits within the pacemaker.) Unfortunately, this positive voltage at the source of the P-channel MOSFET switch has the same result as applying a negative voltage to the gate thereof, i.e., the switch is turned on. This turn on allows the positive voltage to be applied to the input of the telemetry circuits. Because this positive voltage typically exceeds the useful and safe operating range of the telemetry circuits, the telemetry circuits may respond by: (1) telemetering meaningless information (noise); (2) becoming saturated to the point where they are inoperable for a significant time period (the recovery period); or (3) at worst, becoming permanently damaged. Hence, there is a need in the art for protecting such telemetry circuits within a pacemaker that are coupled to a pacing/sensing lead through a P-channel MOSFET switch.